1. Field of the Invention
The present invention relates to method of producing a ceramic green sheet used for manufacturing a multilayer ceramic electronic part such as a monolithic ceramic capacitor, a multilayer varistor, or the like. The present invention also relates to a multilayer ceramic electronic part, particularly to a method of manufacturing a multilayer ceramic electronic part having a structure in which a plurality of internal electrodes are disposed with ceramic layers provided therebetween.
2. Description of the Related Art
Although a monolithic ceramic capacitor which is a typical example of multilayer ceramic electronic parts is widely used for various applications, the monolithic ceramic capacitors have, with progress of miniaturization of electronic parts, recently increasingly been required to have a small size and large capacity.
The monolithic ceramic capacitor has a structure, for example, shown in FIG. 1, in which a ceramic element 3 comprises a plurality of internal electrodes 2a and 2b opposed to each other with ceramic layers serving as dielectric layers provided therebetween, ends of the internal electrodes 2a and 2b being led to different side ends of the ceramic element 3, and a pair of external electrodes 4a and 4b are disposed at both sides of the ceramic element 3 to be connected to the internal electrodes 2a and 2b, respectively.
Ceramic green sheets used for manufacturing the monolithic ceramic capacitor are used for forming the above-described ceramic layers serving as the dielectric layers, and the ceramic green sheets have recently increasingly been thinned in order to increase the acquirable capacitance and decrease the product size.
Each of the green sheets is generally produced by forming ceramic slurry in a sheet shape and then drying. As the method of forming ceramic slurry in a sheet, various methods such as a doctor blade method, a reverse roll coater method and the like, are used.
For example, FIG. 2 shows an example of conventional methods of producing ceramic green sheets, in which a carrier film 52 is supplied from a carrier film supply unit (carrier film supply roller) 51, and a ceramic slurry 54 is coated on the carrier film 52 by a ceramic slurry coater (sheet former) (in this example, using the doctor blade method) 53 at a predetermined position, and conveyed together with the carrier film 52 to a drier 55 for drying the ceramic slurry 54 to form a ceramic green sheet 56. Then, the carrier film 52 holding the ceramic green sheet 56 formed on the surface thereof is wound by a sheet recovery roller 57 so that the formed ceramic green sheet 56 held by the carrier film 52 is recovered.
In the monolithic ceramic capacitor shown in FIG. 1, when the ceramic layers 2 interposed between the internal electrodes 2a and 2b have a thickness (element thickness) of 3 μm or less, thin ceramic green sheets must be produced as ceramic green sheets used for the ceramic layers. However, in producing thin ceramic green sheets by the above-described conventional method, there are the problems of roughening the surfaces of the ceramic green sheets and producing pinhole defects (pores) in the ceramic green sheets.
Furthermore, ceramic dielectric materials such as barium titanate, strontium titanate, calcium titanate and the like, which have a perovskite structure, are conventionally widely used as capacitor materials to utilize their high relative dielectric constants. Also, as a capacitor which is a passive part, a small capacitor which can obtain high capacitance has recently been demanded with miniaturization of electronic parts.
A conventional monolithic ceramic capacitor using a ceramic dielectric material for a dielectric layer must be burned at a high temperature of about 1300° C. in air, and thus a noble metal such as palladium must be used as an internal electrode material. However, such a noble metal material is very expensive and increases the cost ratio of the electrode material in a product. This is a main factor which hinders a decrease in cost.
Therefore, in order to solve the problem, the internal electrodes of the monolithic ceramic capacitor are increasingly made of a base metal material, and various dielectric materials which can be burned in a neutral or reducing atmosphere and which have reduction resistance are developed for preventing oxidation of electrodes during burning.
Under these conditions, the monolithic ceramic capacitor is required to have a smaller size and larger capacity, leading to the progress of development in techniques for increasing the dielectric constant of a ceramic dielectric material and thinning ceramic dielectric layers and internal electrode layers.
However, when each of the ceramic layers (the ceramic layers interposed between the internal electrodes) has a thickness (element thickness) of 3 μm or less, unevenness in the interfaces between the ceramic dielectric layers and the internal electrode layers is increased, or defects (pores) in the ceramic dielectric layers are increased, thereby causing the problem of decreasing the lifetime.
In order to improve the smoothness of a ceramic green sheet used for forming the ceramic layer, and increase the density of the ceramic green sheet, a method has been disclosed in which the particle diameter of a ceramic powder material is decreased (Japanese Unexamined Patent Publication No. 10-223469).
However, as the particle diameter decreases, generally, the ceramic powder itself readily agglomerates to decrease dispersibility. Therefore, the method of decreasing the particle diameter has limits of improvement in the surface smoothness of the ceramic green sheet and increase in the density thereof. For a ceramic dielectric powder having the same composition, the dielectric constant decreases as the particle diameter decreases, thereby causing the problem of making it impossible to comply with a large-capacity monolithic ceramic capacitor.
One method of dispersing ceramic slurry comprises rotating the ceramic slurry at a high speed by a sand mill or visco mill to supply great shear force to the ceramic slurry. However, although dispersion proceeds by applying the shear force to the ceramic powder, this method has the problem of partially grinding the ceramic powder. Grinding the ceramic powder improves the surface smoothness of a sheet due to an increase in dispersibility, but characteristics are changed by grinding the ceramic powder to cause the problem of deviating the temperature characteristics of the resultant monolithic ceramic capacitor from the desired range of design, and decreasing the dielectric constant of the ceramic dielectric.
Furthermore, the number of the layers laminated must be increased while decreasing the size of the monolithic ceramic capacitor and increasing the capacity thereof, and thus a monolithic ceramic capacitor is developed in which about 300 thin ceramic green sheets having an element thickness of 3 μm or less are laminated.
However, with not less than 300 layers laminated, the step formed by the thickness of an internal electrode is increased to cause the problem of producing delamination due to the step of the electrode, and the problem of bending electrode portions being extended for connecting the internal electrodes to external electrodes and producing a short-circuit defect.
These problems apply to not only the monolithic ceramic capacitor but also other multilayer ceramic electronic parts.